A Multiple Input Multiple Output (MIMO) scheme refers to a scheme using multiple transmit antennas and multiple reception antennas so as to improve data transmission/reception efficiency, unlike a conventional scheme using one transmit antenna and one reception antenna. That is, the MIMO scheme increases capacity or improves performance by using multiple antennas in a transmitter or a receiver of a wireless communication system. The MIMO scheme is referred to as a multi-antenna scheme.
MIMO technology collects and combines data fragments received via several antennas without using a single antenna path, in order to receive a message. According to the MIMO technology, data transmission rate can be improved in a specific range or a system range can be increased with respect to a specific data transmission rate. That is, the MIMO technology is next-generation mobile communication technology which can be widely used in a User Equipment (UE), a repeater and the like for mobile communication. This technology is attracting considerable attention as technology capable of overcoming a limit in transfer size of mobile communication due to data communication expansion.
FIG. 1 is a diagram showing the configuration of a general MIMO system. As shown in FIG. 1, if the number of transmitters is increased to NT and, at the same time, the number of receivers is increased to NR, a channel transmission capacity is theoretically increased in proportion to the number of antennas, unlike the case where a plurality of antennas is used in only a transmitter or a receiver. Accordingly, it is possible to improve a transmission rate and to remarkably improve frequency efficiency. As the channel transmission capacity is increased, the transmission rate may be theoretically increased by a product of a maximum transmission rate R0 upon utilization of a single antenna and a rate increase ratio Ri of Equation 1.Ri=min(NT,NR)  Equation 1
For example, in a MIMO system using four transmit antennas and four reception antennas, it is possible to theoretically acquire a transmission rate which is four times that of a single antenna system. After the theoretical increase in the capacity of the MIMO system was proved in the mid-1990s, various technologies of substantially improving a data transmission rate have been actively developed up to now. In addition, several technologies are already applied to the various radio communication standards such as the third-generation mobile communication and the next-generation wireless local area network (LAN).
According to the researches into the MIMO antenna up to now, various researches such as researches into information theory related to the computation of the communication capacity of a MIMO antenna in various channel environments and multiple access environments, researches into the model and the measurement of the radio channels of the MIMO system, and researches into space-time signal processing technologies of improving transmission reliability and transmission rate have been actively conducted.
The communication method of the MIMO system will be described in more detail using mathematical modeling. As shown in FIG. 1, it is assumed that NT transmit antennas and NR reception antennas are present. In transmitted signals, if the NT transmit antennas are present, the number of pieces of maximally transmittable information is NT. The transmitted information may be expressed by a vector shown in Equation 2.s=└s1,s2, . . . , sNT┘T  Equation 2
The transmitted information s1, s2, . . . , sNT may have different transmit powers. If the respective transmit powers are P1, P2, . . . , PNT, the transmitted information with adjusted powers may be expressed by a vector shown in Equation 3.ŝ=[ŝ1,ŝ2, . . . , ŝNT]T=[P1s1,P2s2, . . . , PNTsNT]T  Equation 3
In addition, ŝ may be expressed using a diagonal matrix P of the transmit powers as shown in Equation 4.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    Equation        ⁢                                  ⁢        4            
Considers that the NT actually transmitted signals x1, x2, . . . , xNT are configured by applying a weight matrix W to the information vector ŝ with the adjusted transmit powers. The weight matrix serves to appropriately distribute the transmitted information to each antenna according to a transport channel state, etc. Such transmitted signals x1, x2, . . . , xNT may be expressed by using a vector X as shown in Equation 5. Wij denotes a weight between an i-th transmit antenna and j-th information. W is also called a weight matrix or a precoding matrix.
                                                        x              =                            ⁢                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        i                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                                                                        =                            ⁢                                                                    [                                                                                                                        w                            11                                                                                                                                w                            12                                                                                                    …                                                                                                      w                                                          1                              ⁢                                                              N                                T                                                                                                                                                                                                                                      w                            21                                                                                                                                w                            22                                                                                                    …                                                                                                      w                                                          2                              ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  w                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            w                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      w                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  w                                                                                          N                                T                                                            ⁢                              1                                                                                                                                                            w                                                                                          N                                T                                                            ⁢                              2                                                                                                                                …                                                                                                      w                                                                                          N                                T                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                                                      s                              ^                                                        1                                                                                                                                                                                                          s                              ^                                                        2                                                                                                                                                ⋮                                                                                                                                                                                s                              ^                                                        j                                                                                                                                                ⋮                                                                                                                                                                                s                              ^                                                                                      N                              T                                                                                                                                            ]                                                  =                                                      W                    ⁢                                          s                      ^                                                        =                  WPs                                                                                        Equation        ⁢                                  ⁢        5            
If the NR reception antennas are present, respective received signals y1, y2, . . . , yNR of the antennas are expressed by a vector as shown in Equation 6.y=[y1,y2, . . . , yNR]T  Equation 6
If channels are modeled in the MIMO communication system, the channels may be distinguished according to transmission/reception antenna indexes. A channel from the transmit antenna j to the reception antenna i is denoted by hij. In hih, it is noted that the index of the reception antenna precedes the index of the transmit antenna in view of the order of indexes.
Such channels may be also combined and expressed in the form of a vector and a matrix. A vector expression example is as follows. FIG. 2 shows channels from NT transmit antennas to a reception antenna i.
As shown in FIG. 2, the channels from the NT transmit antennas to the reception antenna i may be expressed as follows.hiT=[hi1,hi2, . . . , hiNT]  Equation 7
Accordingly, through the matrix expression shown in Equation 7, all the channels from the NT transmit antennas to the NR reception antennas may be expressed by Equation 8.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              Equation        ⁢                                  ⁢        8            
An Additive White Gaussian Noise (AWGN) is added to the actual channels after the above channel matrix H. The AWGN n1, n2, . . . , nNR added to the NR reception antennas may be expressed by a vector as shown in Equation 9.n=[n1,n2, . . . , nNR]T  Equation 9
The received signals obtained through the above-described equations may be shown in Equation 10.
                                                        y              =                            ⁢                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                  N                          R                                                                                                                    ]                                                                                        =                            ⁢                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                    …                                                                                                      h                                                          1                              ⁢                                                              N                                T                                                                                                                                                                                                                                      h                            21                                                                                                                                h                            22                                                                                                    …                                                                                                      h                                                          2                              ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                                                                                        =                            ⁢                              Hx                +                n                                                                        Equation        ⁢                                  ⁢        10            
Meanwhile, the number of rows and columns of the channel matrix H indicating the channel state is determined by the number of transmission and reception antennas. The number of rows of the channel matrix H is equal to the number NR of reception antennas and the number of columns thereof is equal to the number NT of transmit antennas. That is, the channel matrix H is an NR×NT matrix. The rank of the matrix is defined by the smaller of the number of rows or columns, which is independent from each other. Accordingly, the rank of the matrix is not greater than the number of rows or columns. The rank of the channel matrix H is expressed by Equation 11.rank(H)≦min(NT,NR)  Equation 11
In the MIMO system, a precoding scheme provides beamforming gain and diversity gain to a transmitter and a receiver so as to provide high system throughput. However, the precoding scheme should be appropriately designed in consideration of antenna settings, channel environment, system structure, etc.
In general, the MIMO system for performing precoding in order to minimize complexity and control signaling overhead uses a codebook based precoding scheme. A codebook includes a predetermined number of precoding vectors/matrices predefined between a transmitter and a receiver according to a transmission rank and the number of antennas. The transmitter selects a specific precoding vector/matrix from a codebook according to channel state information received from the receiver, performs precoding with respect to a signal to be transmitted, and transmits the signal. In some cases, the transmitter may select a precoding matrix according to a predetermined rule, perform precoding, and transmit a signal, without receiving channel state information from the receiver.
FIG. 3 is a diagram illustrating the basic concept of a codebook based precoding scheme.
If a codebook based precoding scheme is used, as described above, a transmitter and receiver share codebook information including a predetermined number of precoding matrices predefined according to a transmission rank and the number of antennas. The receiver may measure a channel state through a received signal and feed preferred precoding matrix information back to the transmitter based on the codebook information. Although FIG. 3 shows the case where the receiver transmits the preferred precoding matrix information to the transmitter on a codeword-by-codeword basis, the present invention is not limited thereto.
The transmitter which receives the feedback information from the receiver may select a specific precoding matrix from the codebook based on the received information. The transmitter which selects the precoding matrix may perform precoding by multiplying layer signals corresponding in number to the number of transmission ranks by the precoding matrix and transmit the signal subjected to precoding via a plurality of antennas. The receiver which receives the signal precoded and transmitted by the transmitter may perform inverse processing of the precoding performed by the transmitter so as to restore the received signal. In general, since the precoding matrix satisfies the condition of a unitary matrix U such as U*UH=I, inverse processing of the precoding may be performed by multiplying a Hermitian matrix PH of the precoding matrix P used for precoding of the transmitter by the received signal.
In a 3rd generation partnership project long term evolution (3GPP LTE) release 8 system, if a MIMO scheme is applied to uplink signal transmission from a user equipment (UE) to a base station, due to deterioration in a peak-to-average ratio (PAPR)/cubic metric (CM) property, a MIMO transmission scheme is defined only with respect to downlink signal transmission from a base station to a UE. A method of applying a MIMO scheme to an uplink signal transmitted from a UE to a base station has been discussed in order to increase a transmission rate and acquire diversity gain. Even in the subsequent standard (3GPP LTE-Advanced or 3GPP LTE-A) of the 3GPP LTE system, a detailed method of applying a MIMO scheme to uplink signal transmission has been discussed.
A UE which supports multiple antenna transmission in an uplink system according to the 3GPP LTE-A standard may operate according to a single antenna transmission scheme or a spatial multiplexing transmission scheme. The single antenna transmission scheme is required when the UE operates in the existing 3GPP LTE system. In this case, the UE combines a plurality of transmit antennas using an appropriate precoding matrix such that the BS recognizes that the UE performs single antenna transmission. Meanwhile, in the spatial multiplexing transmission method using multiple antennas, as described above, it is important to design a precoding matrix to have a low PAPR/CM.